Fuel cell electrode plate assemblies will typically include a porous, thin carbon substrate plate onto one surface of which there is deposited a catalyst layer. The catalyst layer will terminate short of the edges of the substrate plate so that there will be a carbon margin surrounding the catalyst layer. There will also typically be an electrolyte reservoir plate on the side of the substrate opposite the catalyst layer, which reservoir plate will also be a porous carbon structure which is impregnated with additional electrolyte for resupply of electrolyte to the electrolyte matrix. The reservoir plate may be an integral part of the substrate plate in some cases. Reactant gas flow fields will be formed on the side of the substrate plate opposite the catalyst layer.
In order to ensure that the reactant gases cannot cross over from cathode to anode, or reverse at the edges of the substrate plates outwardly of the catalyst layer, these edges must be sealed in some manner so as to prevent diffusion of the reactant gases therethrough. One way to provide such a seal is to locally densify the margins of the substrate plates and then fill the densified portions with liquid electrolyte. The smaller pore sizes in the densified margin will provide a higher surface tension in the liquid filling those pores, so that high reactant gas pressure will not be able to penetrate the entrapped liquid. Thus a liquid seal or "bubble barrier" is formed through which the reactant gases cannot diffuse. There are a number of suggestions in the prior art as to how the densification of the edges of the substrate plate can be accomplished. One general approach involves the use of a very fine grain filler material suspended on a liquid carrier to form what is referred to as an "ink". The ink is then impregnated into the edges of the substrate plate by dipping the edges into the ink, or by injecting the ink into the plate edges under pressure by screen printing. The liquid carrier is then evaporated off leaving the fine grain material behind to reduce the pore size in the edges of the plate. Another method of densifying the edges of the plate which is disclosed in the prior art, involves forming the plates with initially thickened edges which are subsequently compressed thereby densifying their interstices. U.S. Pat. Nos. 3,867,206, granted Feb. 18, 1975 to Trocciola et al; 4,259,389, granted Mar. 31, 1981 to Vine et al; 4,269,642, granted May 26, 1981 to DeCasperis et al; 4,365,008, granted Dec. 21, 1982 to DeCasperis et al; and 4,652,502, granted Mar. 24, 1987 to Donahue et al disclose details of the aforesaid prior art procedures for densifying substrate plate edges to form gas seals. The disclosures of these patents are incorporated herein by reference.
It is highly desirable to have the densified edge portions of the substrate plates underlap the outer edges of the catalyst layer, and it is essential that the top of the densified substrate edge which is outward of the catalyst layer be coplanar with the top of the catalyst layer. In the procedures of the prior art, the edges of the substrate plates are densified prior to the addition of the catalyst layer to the substrate. This ensures that the catalyst will overlap the densified edges of the plates, but creates two problems. One of the problems relates to the difficulty of diffusing gases through the densified edges. This results in non-uniform catalyst deposition on the densified edges when the catalyst is applied by the cloud deposition process. The other problem relates to the fact that the catalyst layer will form a step on the densified edge because the top of the catalyst layer and the top of the densified edge are not coplanar. In order to remedy this deficiency, a thin filler band of the densifying ink must be applied to the top of the densified edges after the catalyst has been deposited. This filler band top surface is made coplanar with the top surface of the catalyst and then the drying step is repeated. Thus when the edges of the substrate are densified prior to application of the catalyst layer, the resultant electrode assembly is somewhat imperfect at the edges of the catalyst layer, and requires additional post catalyst deposition process steps to add the filler band.